You'd think that Kate and William would have the right to keep anyone away that they wish, but the opposite is most likely true. Keeping up proper invitations are essential when in a royal role.
Another good reason for a babymoon is to give you time to learn how to care for the baby and establish a routine without observers. If you have a lot of company early on, you can feel unnecessarily rattled and undermined, caused by unsolicited advice or criticism. Once you feel confident in caring for your baby, welcoming and handling visitors will be less stressful.
Kate and William may or may not be in charge of physically feeding, changing, or otherwise caring for their baby — but without the chance, their confidence in being able to do it could easily be compromised.
You can see that planning a babymoon is a wonderful idea. So if you're not part of the royal family, take advantage of it and have one!